In several communications systems the data to be transmitted is compressed so that the available bandwidth is used more efficiently. For example, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172 and is incorporated herein by reference. The second, known as MPEG-2, refers to ISO/IEC standards 13818 and is incorporated herein by reference. A compressed digital video system is described in the Advanced Television Systems Committee (ATSC) digital television standard document A/53, and is incorporated herein by reference.
The above-referenced standards describe data processing and manipulation techniques that are well suited to the compression and delivery of video, audio and other information using fixed or variable length digital communications systems. In particular, the above-referenced standards, and other "MPEG-like" standards and techniques, compress, illustratively, video information using intra-frame coding techniques (such as run-length coding, Huffinan coding and the like) and inter-frame coding techniques (such as forward and backward predictive coding, motion compensation and the like). Specifically, in the case of video processing systems, MPEG and MPEG-like video processing systems are characterized by prediction-based compression encoding of video frames with or without intra-and/or inter-frame motion compensation encoding.
In a typical MPEG decoder, predictive coded pixel blocks (i.e., blocks that comprise one or more motion vectors and a residual error component) are decoded with respect to a reference frame (i.e., an anchor frame). The anchor frame is stored in an anchor frame memory within the decoder, typically a dual frame memory. As each block of an anchor frame is decoded, the decoded block is coupled to a first portion of the dual frame memory. When an entire anchor frame has been decoded, the decoded blocks stored in the first portion of the dual frame memory are coupled to a second portion of the dual frame memory. Thus, the second portion of the dual frame memory is used to store the most recent full anchor frame, which is in turn used by a motion compensation portion of the decoder as the reference frame for decoding predictive coded blocks.
To reduce the amount of memory required to implement the above anchor frame memory, it is known to compress (i.e., resize) anchor frame image information prior to storage in the anchor frame memory. To ensure accurate prediction using such resized reference image information, it is necessary to correspondingly resize the prediction motion vectors that will utilize the resized reference image information. Present arrangements providing such resizing of images and related motion vector information do not produce satisfactory results under all conditions. Specifically, present arrangements do not function properly in the presence of field prediction encoded macroblocks including inter-field motion vectors.
Therefore, it is seen to be desirable to provide a method and apparatus that significantly reduces the memory and memory bandwidth required to decode a video image while substantially retaining the quality of a resulting full-resolution or downsized video image. Specifically, it is seen to be desirable to provide such a reduction in memory and memory bandwidth even in the presence of field-predictive encoded macroblocks.